Season
A
season is one of the major divisions of the
year, generally based on yearly periodic changes in weather.
In
temperate and
polar regions generally four seasons are recognized:
spring,
summer,
autumn (
fall), and
winter.
In some
tropical and
subtropical regions it is more common to speak of the
rainy (or wet, or
monsoon) season versus the
dry season, as the amount of
precipitation may vary more dramatically than the average temperature.
In other tropical areas a three-way division into hot, rainy and cool season is used. In some parts of the world, special "seasons" are loosely defined based upon important events such as a
hurricane season,
tornado season,
wildfire season or a
sport season.
|
Diagram of the Earth's seasons as seen from the north. Far right: December solstice |
|
Diagram of the Earth's seasons as seen from the south. Far left: June solstice |
 |
Fig. 1
This is a diagram of the seasons. Note that, regardless of the time of day (i.e. the Earth's rotation on its axis), the North Pole will be dark, and the South Pole will be illuminated; see also arctic winter. In addition to the density of incident light, the dissipation of light in the atmosphere is greater when it falls at a shallow angle. |
The seasons result from the
Earth's
axis being
tilted to its
orbital plane; it deviates by an angle of approximately 23.5
degrees of arc. Thus, at any given time during the summer or winter, one part of the planet is more directly exposed to the rays of the
Sun (see
Fig. 1). This exposure alternates as the Earth revolves in its orbit. At any given time, regardless of season, the
northern and
southern hemispheres experience opposite seasons (see
Fig. 2 and
Month ranges of seasons (below) and
Effect of sun angle on climate).
Seasonal weather fluctuations also depend on factors such as proximity to
oceans or other large bodies of
water,
currents in those oceans,
El Niño/ENSO and other oceanic cycles, and prevailing
winds.
In the temperate and polar regions, seasons are marked by changes in the amount of
sunlight, which in turn often cause
cycles of
dormancy in
plants and
hibernation in
animals. These effects vary with latitude, and with proximity to bodies of water. For example, the South Pole is in the middle of the continent of Antarctica, and therefore a considerable distance from the moderating influence of the southern oceans. The North Pole is in the Arctic Ocean, and thus its temperature extremes are buffered by the presence of all that water. The result is that the South Pole is consistently colder during the southern winter than the North Pole during the northern winter.
 |
Fig. 2
As the Earth revolves around the Sun, the seasons in the northern and southern hemispheres are opposite. |
The cycle of seasons in the polar and temperate zones of one hemisphere is opposite to that in the other. When it is summer in the
Northern hemisphere, it is winter in the
Southern hemisphere, and vice versa, and when it is spring in the Northern hemisphere it is autumn in the Southern hemisphere, and vice versa.
In the
tropics, there is no noticeable change in the amount of sunlight. However, many regions (famously the northern
Indian Ocean) are subject to
monsoon rain and wind cycles.Curiously, a study of temperature records over the past 300 years (David Thompson,
Science, April 1995) shows that the climatic seasons, and thus the
seasonal year, are governed by the
anomalistic year rather than the
tropical year.
In
meteorological terms, the
winter solstice and
summer solstice (or the date maximum/minimum
insolation) do not fall in the middle of winter and summer respectively. The heights of these seasons occur up to a month later due to
seasonal lag. Seasons though, are not always defined in meteorological terms;
see reckoningAlso, even though the
Earth's distance to the sun varies as Earth moves between the
apsides of its
elliptical orbit, these variations are not the cause of the seasons.
[PhysicalGeography.net, Fundamentals of physical geography, Ch.6:Energy and Matter:(h) Earth-Sun Geometry, [1]]Besides axial tilt, other factors contribute to seasonal temperature changes. Orbital eccentricity can influence temperatures, but on Earth, this effect is small and is more than counteracted by other factors; research shows that the Earth as a whole is actually a few degrees warmer when
farther from the sun.
[Phillips, Tony, "The Distant Sun (Strange but True: the Sun is far away on the 4th of July)," Science@NASA, downloaded 24 June 2006] Mars however experiences wide temperature variations and
violent dust storms every year at
perihelion.
[Christian Ho, Nasser Golshan, and Arvydas Kliore, Radio Wave Propagation Handbook for Communication on and Around Mars, JPL Publication 02-5, pp. 59-60, downloaded 23 June 2006] |
Illumination of the earth during various seasons |
A common misconception is that, within the
Arctic and
Antarctic Circles, the sun rises once in the spring and sets once in the fall; thus, the day and night are erroneously thought to last uninterrupted for 183 calendar days each. This is true only in the immediate region of the poles themselves.
What does happen is that any point north of the Arctic Circle or south of the Antarctic Circle will have one period in the summer when the sun does not set, and one period in the winter when the sun does not rise. At progressively higher latitudes, the periods of "
midnight sun" (or "midday dark" for the other side of the globe) are progressively longer. For example, at the military and weather station called
Alert on the northern tip of
Ellesmere Island,
Canada (about 450
nautical miles or 830
km from the
North Pole), the sun begins to peek above the horizon in mid-February and each day it climbs a bit higher, and stays up a bit longer; by
21 March, the sun is up for 12 hours. However, mid-February is not first light. The sky (as seen from Alert) has been showing
twilight, or at least a pre-dawn glow on the horizon, for increasing hours each day, for more than a month before that first sliver of sun appears.
In the weeks surrounding
21 June, the sun is at its highest, and it appears to circle the sky without ever going below the horizon. Eventually, it does go below the horizon, for progressively longer and longer periods each day until, around the middle of October, it disappears for the last time. For a few more weeks, "day" is marked by decreasing periods of twilight. Eventually, for the weeks surrounding
21 December, nothing breaks the darkness. In later winter, the first faint wash of light briefly touches the horizon (for just minutes per day), and then increases in duration and pre-dawn brightness each day until sunrise in February.
|
The four season mahjong tiles on the right and the four flower tiles on the other side. The flower tiles are arranged in accord to their growing seasons. |
The date at which each season begins depends on how it is defined. In the
United States, the seasons are often considered to begin at the astronomical solstices and equinoxes: these are sometimes known as the "astronomical seasons". By this reckoning, summer begins at
summer solstice, winter at
winter solstice, spring at the
vernal equinox and autumn at the
autumnal equinox.
The
United Kingdom has no hard and fast rules about seasons, but most follow the equinoxes and solstices; However informally many people use three calendar months for each season with Spring being March, April and May, etc. It is interesting to note that historically seasons were considered to begin about seven weeks earlier than the astronomical seasons: spring began on
Candlemas, summer on
May Day, autumn on
Lammas, and winter on
All Hallows. Accordingly,
midsummer and
midwinter were, as their names suggest, the middle of summer and winter.
The
Irish calendar uses almost the same reckoning; Spring begins on
February 1 /
Imbolc, Summer on
May 1 /
Beltane, Autumn on
August 1 /
Lughnasadh and Winter on
November 1 /
Samhain.
In
meteorology for the Northern hemisphere, spring begins by convention on
March 1, summer on
June 1, autumn on
September 1 and winter on
December 1. This definition is also followed in
Denmark and former
USSR.Conversely, for the Southern hemisphere, meterological summer begins on
December 1, autumn on
March 1, winter on
June 1 and spring on
September 1. This definition is also followed in
Australia.
The
Korean,
Chinese, and
Japanese calendars are based on a
lunisolar calendar, where the solstices and equinoxes mark the middle of each season. This is very close to the British & Irish definitions of seasons. See below,
using solstices and equinoxes as mid-days.
Month ranges of seasons| Northern Hemisphere | Month | Southern Hemisphere |
|---|
| Traditional | Meteorological | Astronomical | Meteorological | Astronomical |
|---|
| Winter | Winter | Winter | January | Summer | Summer |
|---|
| Spring | February |
|---|
| Spring | March | Autumn |
|---|
| Spring | April | Autumn |
|---|
| Summer | May |
|---|
| Summer | June | Winter |
|---|
| Summer | July | Winter |
|---|
| Autumn | August |
|---|
| Autumn | September | Spring |
|---|
| Autumn | October | Spring |
|---|
| Winter | November |
|---|
| Winter | December | Summer |
|---|
Mid-season
In the conventional US calendar, the following dates are considered to be halfway through a season:
*Winter (
February 3)
*Spring (
May 5 or
May 6)
*Summer (
August 7)
*Autumn (
November 6)
Using solstices and equinoxes as mid-days
With the solstices (peaks) are used as the middle of each season, as is in
Jieqi and East Asian countries, the longest day of the year occurs in the middle of summer, and conversely the shortest day of the year in the middle of winter.
So, in the Northern Hemisphere:
*Spring begins (
February 3)
*Summer begins (
May 5 or
May 6)
*Autumn begins (
August 7)
*Winter begins (
November 6)
In the Southern Hemisphere:
*Autumn begins (
February 3)
*Winter begins (
May 5 or
May 6)
*Spring begins (
August 7)
*Summer begins (
November 6)
In
hemiboreal and
temperate climates:
Image:Schneelandschaft_Furx.JPG|In the winter, the plants are at rest.Image:Urban spring blooms.jpg|In the spring, the plants start to grow again.Image:The Chase Wood - Newbury.jpg|In the summer the plants grow.Image:Czechia, Jicin, Wallenstein's alley.jpg|In the autumn, the trees turn yellow and drop their leaves.*
List of songs about the seasons of the year*
When do the Seasons Begin? (from the
Bad Astronomer)
*
Solstice does not signal season's start (from
The Straight Dope)
*
Why the Earth has seasons article on
h2g2.
